Water heaters for supplying hot water at a bath room and a kitchen are roughly classified into electric water heaters, gas water heaters, oil water heaters, and the like. Any of the types of water heater includes a part called “heat exchanger” for transferring heat to water.
Among the electric water heaters, a heat-pump heat-exchange type electric water heater (heat-pump water heater) has recently attracted particular attention in view of reduction of carbon dioxide as measures to save energy and countermeasures against global warming.
The heat-pump water heater operates on the principle that heat in the atmosphere is transferred to a heat medium to heat water with the transferred heat. More specifically, the heat-pump water heater is operated by a repeated operation (cooling-heating cycle) of transferring high heat, which is generated when a gas is compressed, to water through an intermediation of the heat exchanger, and then lowering the temperature of the heat medium back to an atmospheric temperature with cool air generated when the gas is expanded. Even though a larger amount of heat energy than that of input energy cannot be obtained in theory, a larger amount of heat energy than the amount of energy required for the operation can be used because the heat-pump water heater has a mechanism of using the heat in the atmosphere.
It is very important to constantly maintain a heat-transfer surface in a clean state because the heat exchanger transfers the heat to the water. When a wall surface is stained, an effective heat-transfer area is reduced, which in turn lowers heat-transfer performance. When the stain is further accumulated, a flow channel is obstructed in the worst case.
In a region where a large amount of hardness components (calcium ions or magnesium ions) is contained in water, in particular, there is a problem in that carbonate crystal called “scale” is deposited by heating and is likely to adhere to the inner side of the heat exchanger.
As a water heater for solving the problem described above, there is known a water heater including means for generating a nucleus of a scale component, which is provided in a stage previous to the heat exchanger, during the heat exchange, in which the heat exchanger is partially formed of a base material made of copper or copper alloy, and at least a surface of the base material which may come into contact with water during use is coated (for example, see Patent Literature 1).
In the case of the water heater described above, the nucleus generation means includes a plurality of electrodes and a power supply for applying a voltage between the electrodes. A pH of water in the vicinity of a cathode is increased as a result of electrolysis of the water, and hence a nucleus of calcium carbonate crystal is generated in the vicinity of the cathode. As a result, Ca is unlikely to be brought into an oversaturated state on a surface of the heat exchanger. Therefore, an insoluble Ca compound is unlikely to be deposited.